Have to agree with bc... Don't know if this is much help. We have the same problem off and on. We use drain cleaner and that seems to make it worse. In the end we rent a snake and run it in our outside clean out pipe I put in a few years ago. Tree roots fine as hair. Check the upper part of the bowel making sure that water is running out of the holes under the top by the seat. Check to see if the water has built up water deposits as some water is on the hard side and can do a real number in the bowl and in the tank. ( stains sides of bowl) We have city water and regular street drain system. Septic tanks need to be cleaned on a regular bases. Root X and other such products are a help. If you have plastic pipes be sure cleaner is safe for it. Drop in a dye tablet that you can get at most hardware places, this will help check for leaks and you can see if the upper holes around the bowl are clear by looking for dye lines running down the bowl. As said check all parts and adjustments before running out and spending your money.